Using the Lenstar Optical Biometer in Small Animal Research with A-Scan Ultrasonography-Matched Refractive Indices

Abstract

Purpose : The Lenstar uses a near infrared light source to measure optical path lengths (OPLs), which are converted into geometrical lengths (GLs) based on human eye refractive indices. We aim to estimate refractive indices for the tree shrew eye that can be used to properly convert the Lenstar OPLs into realistic GLs for this small animal.

Methods : Non-cycloplegic axial measurements were performed using A-scan ultrasonography (ASUS) and the Lenstar optical biometer in 80 juvenile tree shrews. A total of 188 paired measurements were performed using each technique. Normal eyes and eyes with experimentally induced refractive errors at varying degrees of myopia and hyperopia were included. Animals were anesthetized for ASUS while LenStar measurements were performed in awake animals. GLs obtained from ASUS represent our reference data. OPLs were exported from the Lenstar software and converted into GLs. The refractive indices that are needed for this conversion were estimated for each ocular segment by matching the converted GLs to the GLs obtained from ASUS.

Results : Our Lenstar instrument operates at a wavelength of 831nm. For this wavelength, the ASUS-matched refractive indices were estimated to 1.3868, 1.3963, and 1.3672 (R = 54%, 78%, 73%) for the anterior segment, lens, and vitreous, Respectively. Compared to the ASUS data, the anterior segment depth and vitreous chamber depth were significantly underestimated when using the Lenstar internal analysis. This error was significantly reduced when using the ASUS-matched refractive indices to compute the Lenstar GLs (Figure).

Conclusions : The internal analysis performed by the Lenstar software leads to erroneous results when used for small animal eyes. We estimated species-specific refractive indices that allow for realistic conversions of Lenstar measured OPLs into GLs in tree shrews. A similar conversion can be established for other species.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Figure: Box plots for the anterior segment depth, lens thickness, and vitreous chamber showing the difference between our reference results (ASUS) and the Lenstar results when using the Lenstar internal analysis or the ASUS-matched refractive indices.